Quantum well behavior without confining barrier observed via dynamically screened photon field

Angle-resolved photoemission spectra from Na adlayers on Al(111) reveal fea tures which behave like quantum well resonances although the substrate prov ides no confining barrier. These features are observed in a narrow photon e nergy range where overlayer collective excitations cause resonant enhanceme nt of the photoemission intensity. The quantum well behavior is shown to be due to surface resonances of the Na/Al system. The resonances are observab le using photoemission because of spatial confinement and dynamical enhance ment of the local electric held within the: Na films.